1. Field of the Invention
The present invention relates to a direct-current resistance welding apparatus for controlling a plurality of welding guns with a single inverter circuit and a welding timer circuit, and also for pressing the welding guns against a workpiece and energizing the welding guns according to independent control processes, and a method of controlling the welding current of the direct-current resistance welding apparatus.
2. Description of the Prior Art
Some known direct-current resistance welding apparatus have a plurality of welding guns that are supplied with welding currents by a single inverter circuit and a welding timer circuit. There are known two different processes for controlling the welding guns. According to the first process, a welding gun is controlled to grip a workpiece and then supply a welding current to the workpiece. Thereafter, another welding gun is controlled to grip a workpiece and then supply a welding current to the workpiece. Such a two-step operation is carried out successively with respect to all the welding guns. In the second process, all the welding guns are simultaneously controlled to grip respective workpieces, and then the welding guns are successively controlled to supply welding currents to the workpieces.
The first process requires a circuit to prevent the welding guns from gripping the workpieces simultaneously for thereby preventing a welding current, which has been supplied to a workpiece, from flowing to other workpieces. The second process needs either a switching switch connected to the primary winding of a welding transformer or a current distributor connected to the secondary winding of the welding transformer for preventing a welding current from flowing to other welding guns, as disclosed in Japanese Laid-Open Utility Model Publications Nos. 55-92485 and 63-133884.
According to the first process, while a workpiece is being gripped by a welding gun, the inverter circuit is inactivated, and while the inverter circuit is in operation, all presser mechanisms coupled to the welding guns are at rest. Therefore, the direct-current resistance welding apparatus that are controlled according to the first process are inefficient in operation. Moreover, the circuit to prevent the welding guns from gripping the workpieces simultaneously makes the direct-current resistance welding apparatus complex in structure and large in size.
The direct-current resistance welding apparatus that operate according to the second process are also complex in structure and large in size because of either the switching switch connected to the primary winding of the welding transformer or the current distributor connected to the secondary winding of the welding transformer. Another problem is that a relatively large voltage drop is developed by the current distributor.